Apparatus for burying pipeline, cable and the like have been well known in the art for many years. Essentially, they incorporate some means of jetting water in advance of the movement of the apparatus to dig and cut away a trench into which the pipeline is to rest. They further include a means for guiding and driving the apparatus along the pipeline to be buried, a buoyancy tank system which lifts the apparatus up so that its entire weight is not resting on the pipeline, and an eductor system for sucking up the cuttings and the other debris formed in the trench by the cutting means and discharging them away from the trench. Examples of such prior systems include U.S. Pat. Nos. 3,926,003, 3,877,237, 4,087,981 and 4,389,139. However, it has been found to be desirable to improve the efficiency of these prior art machines and to rectify some of the problems as will be discussed.
In particular, it has been found to be desirable to improve the traction exerted by the drive rollers on the pipeline and thereby to increase the speed that the apparatus moves down the pipeline for a given amount of input energy. Drive rollers in the past were typically V-shaped or hour-glass-shaped in which there was only a minimum contact interface between the pipe and the individual roller. These prior roller designs also provided for a generally flat or flat curved surface which was not in continuous contact with the pipe as it moved along the pipe.
One design for the cutting means provided for a downwardly depending pair of jet tubes each jet tube having a plurality of jets on its forward and side surfaces. Each of these jets, however, individually was unable to focus a large force of water against the sea bed. Also, inasmuch as the jets were not symmetrically disposed about the entire jet tube an undesirable backward reactive force was exerted. This force created a force on the jet tube as well as a backward force reacting against the driving motion of the drive rollers requiring greater traction. The jets further would cut the trench but would not effectively disperse the cuttings up and away from the trench to thereby aid the eductors in removing the cuttings from the trench.
The drive roller assemblies constituted an integral part of the frame in past designs. The mounting for the roller and the motor was secured to a vertical shaft that was welded into the frame in place. Therefor, when it was desired to replace a roller which had become damaged or worn, each damaged or ineffective unit would have to be removed sometimes taking many hours to replace the roller or the motor, etc.
Buoyancy systems for underwater trenching apparatuses have comprised a pair of buoyancy tanks rigidly mounted on opposite sides of the frame. Each of the tanks would include two or more noncommunicating vertical compartments disposed side by side. When it was desired to effect a torque on the frame to compensate for the torque exerted by underwater cross current, it was necessary to pump air into or out of one or more of the compartments on one or both sides of the apparatus. This was a complicated procedure and had the undesirable effect in that the overall buoyancy effect of the buoyancy tanks would be altered when it was desired to alter the torque effect that the buoyancy tanks exerted on the frame. Also, complicated pumping and valve mechanisms were required for each compartment in order to adjust the buoyancy effect of that compartment.
Eductors found in the prior art machines had been ineffective in excavating the ditch because of the turbulence of the water containing the cuttings surrounding the machine. The eductors were not efficient in that the cuttings do not settle until the machine has well passed beyond the initial point of cutting and they cannot be thoroughly discharged until they settle toward the bottom of the trench.
It is frequently desirable to have the machine make several passes along the pipe in order to dig a trench of the proper depth. In the past, typically it was a complicated procedure to either turn the machines around on the pipe or to cause a reversal of the motions of the individual components.